Programming Assignment 1

User interfaces, as we have learned, are not part of most operating systems (although they were in very early systems). Still they are very important, since users rely on them for everyday use of the system.

Text-oriented, or command line user interfaces are frequently called “shells.” It is somewhat interesting that a very popular shell for the UNIX operating system was developed by an individual named Donald Shell. The default shell we use with OS/390 Unix System Services is the Bourne shell, named after its creator.

The C source code for a very simple shell can be found in /u/cs450/unosh.c. We will examine this source code in class, and note its use of the system calls read, write, fork, and execve. This shell prints a prompt on the user’s terminal, reads a command line consisting of a program name and any additional parameters. It then parses the command into words (sequences of non-blank characters), and creates a process to execute the program. The created process then executes the appropriate system call to replace the code it is currently executing (in the shell) with the named program, passing it any parameters given on the command line. The original process then repeats (by printing a prompt, etc.) until the end of file on the standard input is encountered.

You should first copy the source code for this shell to your own directory, compile it, then execute it using a few standard commands as input. If your username was cl31s50, you could copy the shell’s source code using the command

cp /u/cs450/unosh.c /u/cl31s50/unosh.c

You could then compile and link it using the command

cc –o unosh unosh.c

The cc command runs the C compiler, producing a file named unosh.o which contains the object code. If that’s successful (that is, there were no syntax errors), the linker will combine unosh.o and the appropriate libraries to produce the executable file, in this case named unosh. The “-o unosh” part of the command gives an explicit name to the executable file. Without that option (i.e., if you had used the command cc unosh.c) the executable file would have been given the default name a.out.

To run the program, just enter the name of the executable file:

unosh

The prompt (“>>>”) will be printed and the program will wait for you to enter a command. Try entering “ls” or “ps” or “pwd”. Then try “ls –l” to verify that additional parameters are accepted by the shell. You can learn what each of these commands does by using the “man” command. For example, entering “man ls” will give you a description of the ls command. Once you’ve played with unosh, terminate it by entering control-D (i.e. hold the control key down and press D) after a prompt. The control-D character is interpreted by the system as indicating an end-of-file for the file associated with the keyboard, and the program will terminate.

The Assignment

You are to modify this program (or rewrite it completely, if you wish) to allow the user to select a command to be executed from a list of predefined commands that will be given in a text file, and named as the first parameter to your modified shell, which we’ll assume is named mysh. Suppose the text file cmds contains the following lines:

pwd

ls –l

Executing the command “mysh cmds” will first print each of the lines in the cmds file (there will be no more than 9), prefixed by a one digit integer (1 for the first command, 2 for the second, and so forth) and a blank. It will then repeatedly print a prompt, read a line containing a single digit integer (and the end of line character), then execute the corresponding command. When the user enters control-D then mysh will terminate. When the user enters 0 the program will redisplay the list of commands and their associated numbers.

Here’s an illustration of how the use of the program might appear on your terminal, assuming the standard shell prompt is ‘$’. Text appearing in italics shows output from executing a command, and text shown in bold is typed by the user.

$ mysh cmds

1 ps

2 ls

3 pwd

4 ls –l

>>> 1

- output from the ps command would appear here –

>>> 4

- output from the ls –l command would appear here –

>>> 0

1 ps

2 ls

3 pwd

4 ls –l

>>> ^D

$ -- and we're back to the original shell –

Unix text files are just sequences of character codes (in this case, EBCDIC), periodically punctuated by end of line characters. The end of line character is usually represented as the character string '\n' in a C or C++ program. You may assume that the text file will never contain more than nine lines, each of which is never longer than 80 bytes, not including the end of line character.

This program is due on July 11, 2001. Be certain to review the guidelines for submitting assignments before that date, and make certain your source code only appears in the appropriate directory.